The basic theory of sound transmission is set forth, for example, in standard introductory physics textbooks such as Resnick and Halliday, Physics, Part I, John Wiley & Sons, 1977, pp. 433-456. As described therein, sound is a longitudinal mechanical wave having a frequency within a range of approximately 20 Hz to 20 KHz. Typically, sound is generated by vibrating elements which alternately compress the surrounding air on a forward movement and rarefy it on a backward movement. Air transmits these disturbances outward from the source as a longitudinal wave. Upon entering the ear, these waves produce the sensation of sound.
In the art of sound reproduction, a loudspeaker is generally understood to be a device which converts electrical energy into sound energy. Multiple loudspeakers are often used in sound reproduction applications requiring high acoustic power output, and it is also common to provide different devices for reproducing the bass (low-frequency), midrange and high-frequency portions of the sound spectrum. A tweeter is generally responsive only to the higher acoustic frequencies, i.e., frequencies higher than approximately 6 kHz, and reproduces sound of high pitch.
The primary components of a loudspeaker are an electromagnet and a vibrating diaphragm attached to an armature that is vibrated by the variations of electric current in the electromagnet. A cone speaker is a particular type of loudspeaker in which the vibrating diaphragm is relatively large and conical and usually made of paper. A simple cone speaker assembly includes a speaker housing or cabinet, a transducer, and a speaker cone. The transducer causes the speaker cone to vibrate in response to signals from an amplifier, thus producing sound in the manner described above. The vibration of the speaker cone generates two longitudinal sound waves, a front wave and a back wave, which, at least initially, propogate in opposite directions. It is generally the front wave which generates the sounds, such as music, heard by a listener.
In previous sound reproduction systems, the sounds heard by a listener have often been directional in nature and have depended upon the relative positioning of the loudspeaker and the listener. Thus, the loudspeakers in a room must be carefully arranged by a listener to properly direct the sound for maximum acoustic quality. However, even a careful arrangement of speakers within a room is often unsatisfactory since it is unlikely that all of the listeners in a particular room will be positioned so as to be in a region of maximum acoustic quality. This problem of directionality has been particularly bothersome in systems incorporating domed or hemispherical-type tweeters, which are highly directional and which normally tend to project a strong front wave of sound energy in an axial direction.
In some sound reproduction systems, the vibration of the loudspeaker diaphragm also generates subordinate vibrations such as cabinet vibrations, which contribute to low quality sound reproduction. This is particularly true of cabinets formed of acoustically active materials, such as wood, which are relatively sensitive to vibratory forces. These subordinate vibrations modify or "cloud" the sound generated by the excitation of the diaphragm. This effect is known as intermodulation (IM) distortion, and is an important factor to be addressed in speaker design.
Because loudspeakers are designed to reproduce sound as faithfully as possible, the design must be compatible with the physics of sound, particularly if it is desired to reproduce high fidelity musical sound. Musical waveforms have a sinusoidal basis. Since sine waves are algebraic, a sound reproduction device should be curvilinear, rather than rectilinear, in design. An example of such design is the "bell-type" shape of many musical instruments.
All musical sound waves travelling in a medium (such as air, water, etc.) are acoustical (or physical). The sound waves are also algebraic functions since the fundamental frequency is sinusoidal (as on the lowest notes from the flute). All harmonics, or overtones, are also sinusoidal. Since the function of a loudspeaker is to receive the electronic format of the sound and reproduce the sound as acoustical (or physical) wave action in the medium through which it travels, it follows that the shapes involved in the loudspeaker baffle, resonating and transferral devices should be compatible with this sinusoidal nature. Therefore, the most compatible shapes should be curves, spheres, triangles or pyramids, rather than straight lines, cubes, squares or rectangles. The compatible sinusoidal shapes greatly enhance the sinusoidal characteristics of each fundamental frequency and its complement of harmonics, or overtones. This also applies to "ports" which transfer acoustical sound pressure waves within the speaker assembly, and also to the transfer of the waves to the surrounding transmitting medium.